Comment # 8 on bug 928271 from Reinhard Max

AFAIU, it is not a bug, but usually wanted behaviour that PAM logs this kind of
stuff. It only gets annoying when you have cron jobs that run very often, but
then it is up to you to disable the logging.

You can do this by adding the line that was suggested in comment 0 to
/etc/pam.d/common-session and change "cron" in it to "crond", so the line
reads:

session [success=1 default=ignore] pam_succeed_if.so service in crond quiet
use_uid

To silence the pam_unix(systemd-user:session) log lines as well, you can extend
the line to

session [success=1 default=ignore] pam_succeed_if.so service in
crond:systemd-user quiet use_uid

But be aware that the latter might also silence desired log messages coming
from stuff like local logins, su, sudo, or ssh.

... and don't forget to restart crond after changing the PAM configuration.